1. Field of the Invention
The present application relates to an electrochemical energy storage device.
2. Description of the Related Art
In recent years, research and development for increasing the energy density of non-aqueous electrolytic solution rechargeable batteries have been increasingly extensive. The non-aqueous electrolytic solution rechargeable batteries are used as a power supply for electronic devices, such as cell phones, portable information devices, laptop computers, video cameras, handheld game consoles, etc., a power supply for driving electric tools, cleaners, robots, etc., a power supply for driving or supporting an electric motor of hybrid electric cars, plug-in hybrid electric cars, fuel cell powered cars, etc.
To increase the energy density of the non-aqueous electrolytic solution rechargeable battery, it is necessary to use a material which has large electric capacity as an active material of an electrode. For example, when cupric chloride (CuCl2) is used as the positive electrode active material and a lithium metal is used as the negative electrode active material, the electric capacity of 399 mAh/g can be obtained through reactions such as represented by formula (1) and formula (2). This value is equivalent to about three times the electric capacity of Li0.5CoO2 that is used as the positive electrode active material in lithium ion batteries of today. Note that the potentials shown at the heads of formula (1) and formula (2) are values calculated based on the standard free energies of formation of cupric chloride, cuprous chloride (CuCl), and lithium chloride (LiCl).3.40 V: CuCl2+Li+e→CuCl+LiCl  (1)2.74 V: CuCl+Li++e→Cu+LiCl  (2)
Japanese Laid-Open Patent Publication No. 2004-47416 (hereinafter, referred to as “Patent Document 1”) discloses a non-aqueous electrolytic solution rechargeable battery in which cupric chloride (CuCl2) is used for the positive electrode active material, and suggests using a fluorinated solvent as the solvent of the non-aqueous electrolytic solution. Patent Document 1 discloses that, for example, when electric discharge (reduction) of cupric chloride is caused in a non-aqueous electrolytic solution in which lithium hexafluorophosphate (LiPF6) at the concentration of 1 M and lithium chloride (LiCl) at the concentration of 2.4 mM (“100 mg/liter” in Patent Document 1) are dissolved as the electrolyte salts with the use of trifluoropropylene carbonate (abbreviated as TFPC), the reaction of formula (2) occurs subsequent to the reaction of formula (1). Here, the reasons for the use of TFPC are to suppress excessive dissolution of cupric chloride in the electrolytic solution and to prevent self-discharge.